1. Field of the Invention
The invention generally relates to an image system. More particularly, the invention relates to an image system of a projection display apparatus.
2. Description of Related Art
In order that an image system adapted to a projection display apparatus may have a good imaging quality, the image frame projected by the image system may conform to a low aberration, a high resolution, a high contrast ratio, and a high uniformity. Moreover, designing the image system of the visible light and the infrared light focusing on the same one plane usually needs lenses made by a special optical material of dispersion.
However, the refractive index of the special optical material of dispersion obviously decreases with the temperature raising. In other words, the thermal-optical coefficient (equate dn/dt, wherein n is refractive index and t is absolute temperature) of the special optical material of dispersion is negative value, and the absolute value of the thermal-optical coefficient of the special optical material of dispersion is higher than that of a general material. Therefore, a focusing plane of the light may shift.
FIG. 1 is a schematic view of a conventional focusing surface shifting with temperature of an image system. Referring to FIG. 1, a lens 50 of an image system is made by a special optical material of dispersion, and the refractive index of the lens 50 obviously decreases with the temperature raising. In FIG. 1, the transmission path of the light beam 60 is the transmission path of light when the temperature of the lens 50 is 20 degrees Celsius, and the transmission path of the light beam 60′ is the transmission path of light when the temperature of the lens 50 is 40 degrees Celsius. The light beam 60 is focused on a plane 70 and the light beam 60′ is focused on a plane 80. In other words, with the temperature of the lens 50 increasing, a back focal length (BFL) D of the image system is longer so that the imaging quality of the image system becomes worse.